1. Field of the Invention
This invention relates to hydrometallurgical processes and, in particular, to heap leaching copper ore using a solution including sodium nitrate.
2. State of the Art
Chemical processes to obtain copper metal are becoming increasingly important due to their relative simplicity and environmental compatibility, as compared to traditional thermal processes. Heap leaching has the advantages of low investment cost and low operating costs. Presently, there are two principal techniques within the mining industry for treating sulfide ores by heap leaching. These are bacterial heap leaching and acid heap leaching.
Bacterial leaching of copper ore is generally faster than acid leaching because bacterial leaching combines the oxidizing biochemical activity of bacteria with the chemical action of sulphuric acid and oxygen. The process includes irrigating a mineral heap with a dilute solution of sulfuric acid inoculated with a given strain of bacteria. The bacteria used, for example, Thiobacillus ferrooxidans, Leptospirillum ferrooxidans, or others, are cultivated from bacteria naturally occurring on the ore and are essentially strains of bacteria adapted to the specific physical-chemical conditions existing in the environment of a particular ore deposit.
Presently, the most widely used sulphide ore leaching process in industry is bacterial leaching. This process has the advantage of low investment cost, low operational cost, and relatively low final cost per pound of fine copper produced (approximately US$ 0.50/lb at present). This process also has some undesirable characteristics such as a reaction time measured in months (usually six months for recoveries about 60%, nine months for recoveries about 85%) and comparatively low recoveries that fluctuate between 65% and 85% of the metal contained in the ore. The life and proliferation of bacteria depend critically on temperature, acidity, and availability of nutrients.
Acid leaching is potentially more efficient than bacterial leaching but this potential has not been realized in the prior art. Acid leaching is similar to corrosion in that the leaching phenomenon occurs throughout the whole structure of the ore mineral crystals, whereas the leaching by bacteria occurs only where the bacteria are emplaced, that is, only on the surface of the ore mineral crystals in contact with the solution. This difference implies a difference in the number of sites where leaching occurs, which clearly favors acid leaching due to its action at a molecular level throughout the whole structure of the sulphide ore mineral crystals; that is, throughout the whole volume of the crystals that are in contact with the leaching solution.
Heap leaching typically uses sulfuric acid and ferric sulfate as the oxidant in a solution. Ferric sulfate is obtained from decomposition reactions of sulfides (pyrite and chalcopyrite) through the combined action of water dissolved sulfuric acid and oxygen.
U.S. Pat. No. 287,737 (Stetefeldt) discloses treating cakes of ore in a vessel with sulphuric acid and sodium nitrate to recover copper and silver. The NOx gases produced are recovered in a water spray.
U.S. Pat. No. 3,793,429 (Queneau et al.) discloses using nitric acid for recovering copper and other metals from sulphide ore. The NOx gases produced are recovered with cool water and oxygen to regenerate nitric acid.
U.S. Pat. No. 3,912,330 (Carnahan et al.) discloses a process for chemically treating copper ores with a mixture of nitric acid and sulphuric acid with oxygen under pressure and discloses that the process in not suited to heap leaching.
U.S. Pat. No. 5,403,382 (Fairbanks) discloses heap leaching copper ore with sulphuric acid at a pH of 1.5.
To be competitive with bacterial leaching and thermal techniques, acid leaching must be made faster without adversely affecting the environment. The NOx gases produced by the processes described in the Stetefeldt patent and the Carnahan et al. patent make these processes undesirable from an environmental standpoint. The problem is made more acute by the fact that increasing the speed of the reaction process generally causes more NOx gases to be produced, not less. Adding equipment to recover the NOx gases only increases the cost of the process.
It is obviously less expensive to transport copper than to transport copper ore. A problem with prior art processes, particularly the process described in the Carnahan et al. patent is that the site of the ore is typically not at sea level, i.e. oxygen is less abundant and providing additional oxygen, even in the form of compressed air, excessively increases the cost of the process.
In view of the foregoing, it is therefore an object of the invention to provide a faster process for leaching copper ore with acid.
Another object of the invention is to provide a process for leaching copper ore that is cost competitive with other processes.
A further object of the invention is to provide a process for leaching copper ore in which the emission of NOx gases is eliminated without the use of separate recovery apparatus.
Another object of the invention is to provide a process for leaching copper ore to recover at least 85% of the available copper.
A further object of the invention is to provide a process for leaching copper ore without the addition of gaseous oxygen.
The foregoing objects are achieved in this invention in which sodium nitrate is an oxidizing chemical reactive in a sulphuric acid leaching solution. In particular, the ore is crushed to  less than 6 mm and stored in heaps less than twelve meters high. The heaps are irrigated with a dilute solution consisting essentially of sulphuric acid and sodium nitrate to produce a leach product. The leach product is recovered from the heap and copper is recovered from the leach product by electrolysis. In accordance with another aspect of the invention, acid leaching with sodium nitrate is dependent on acidity (pH), which is maintained at xe2x89xa61.7 pH.